This invention relates to a control apparatus for an A.C. elevator for improving the riding quality of a cage at the start thereof.
FIG. 5 is a view showing the conventional construction of a magnet brake which is assembled unitarily with a hoist.
Normally, brake levers 50 are urged in the directions of arrows A by springs 51. In consequence, brake shoes 52 grasp a brake wheel 53 to restrain the rotation thereof. The brake wheel 53 is secured to a rotary shaft 54 which is directly coupled to a motor, and it restrains the rotation of the motor, and in turn, the movement of a cage.
Besides, cams 55 each of which is formed in the shape of the letter L turn in the directions of arrows B with the movements of the brake levers 50 in the directions A, thereby to push up a plunger 56.
When a brake coil 57 is fed with a supply voltage, the plunger 56 is attracted to descend. With the descent, the cams 55 are turned in the directions of arrows C, and the brake levers 50 are moved in the directions of arrows D against the springs 51. Accordingly, the brake shoes 52 release the braked wheel 53. Owing to the release, the rotary shaft 54 is driven by the motor so as to move the cage up or down. Here in the figure, letter X denotes the air gap of the magnetic circuit of the plunger 56. When the plunger 56 is attracted, the inductance of the magnetic circuit increases due to the disappearance of the air gap X.
A prior-art example of a control apparatus for an A.C. elevator employing the above brake, will be explained with reference to FIG. 6. In the figure, numeral 1 indicates an A.C. three phase power source, and numeral 2 an electromagnetic contactor which switches each electric path extending from the A.C. power source 1 and which has a normally-open contact 2a. A drive circuit 3 for the motor 4 is configured of, e.g., thyristors or transistors The motor 4 which is driven by the drive circuit 3, rotates the rotary shaft 54 so as to move the cage 62 up and down.
Numeral 9 designates an electromagnetic contactor which feeds the brake coil 57 with the supply voltage 10, and which has a normally-open contact 9a. A control circuit 11 is actuated by the closure of a start command contact 12, to energize the electromagnetic contactors 2 and 9 and to operate the drive circuit 3. Symbol V.sub.B denotes a control voltage source.
Shown at numeral 60 is a sheave which is coupled to the rotary shaft 54, and round which a main rope 61 is wound to move the cage 62 and a counterweight 63 up and down in a well-bucket fashion. In the above construction, a braking force is generated by deenergizing the brake coil 57, thereby to restrain the cage 62, and the brake coil 57 is energized in accordance with a start command signal, thereby to release the braking force.
In operation, when a call has occurred in the cage 62, the start command contact 12 is closed, and the control circuit 11 is actuated to energize the electromagnetic contactors 2 and 9. Thus, the contacts 2a and 9a are respectively closed to feed the drive circuit 3 with electric power by means of the A.C. power source 1 and simultaneously to energize the brake coil 57 by means of the voltage source 10. Further, an operation command is sent to the drive circuit 3 with aim taken at the timing at which current flows through the brake coil 57 to attract the plunger 56 and to release the brake wheel 53. Then, the drive circuit 3 feeds the motor 4 with electric power so as to generate a torque. The cage 62 is started to ascend or descend by the torque.
The prior-art control apparatus for the A.C. elevator is constructed and operated as stated above. Therefore, when the cage 62 is to be started, the case that the timing of releasing the brake does not coincide with the timing of the supply of the electric power to the motor 4 sometimes occurs and that the motor 4 generates the torque while a braking force is still acting on the brake. In this case, the phenomenon of the rush-up or retrogression of the cage 62 in the start mode arises depending upon the magnitude of the load in the cage and the direction of the movement of the cage, causing the riding characteristics of the cage 62 to worsen.
More specifically, as in the prior-art example show in FIG. 6, the operation of releasing the brake for the elevator is usually performed in such a way that the contact 9a is closed to apply the constant voltage E by means of the D.C. voltage source 10. Then, the coil current i increases depending upon the values of the inductance L and resistance R of the coil 57, as indicated by the following formula: EQU i=E/R[1-exp(-L/R.multidot.t)]
On the other hand, the torque of the brake decreases with the increase of the coil current However, the increase of the brake coil current, which in turn causes the decrease of the braking torque, cannot be favorably controlled merely by applying the constant voltage E, so that the brake is instantly released in most cases. In the start mode of the elevator, accordingly, the cage 62 sometimes starts suddenly or retrogresses due to the difference in weight between the cage 62 and the counterweight 63. Even when the applied voltage E can be selected to the optimum value in the above formula, the resistance R varies due to a voltage fluctuation and a temperature fluctuation, and the increase of the current i cannot be favorably controlled.
In order to avoid such a drawback, there has been ordinarily employed a method wherein the load in the cage is detected and wherein a velocity command is biased in accordance with the detected result of the load and the running direction of the cage. With this method, however, a load detector etc. have an expensive mechanical construction, and the adjustments thereof are laborious.